Thousands of miles of buried natural gas mains of varying sizes and formed from various materials are presently in service. All of these mains are in some state of progressive degradation. In most instances, the extent of such degradation is unknown, and hence, the serviceability of the mains is similarly unknown. This lack of information with respect to degree of degradation results in unforseen gas main breaks and/or leaks, and necessitates the expending of substantial time and expense in locating these defects so that repairs and/or replacement can be made. Because of the need to detect conditions which might result in gas main breaks and/or leaks, apparatus has been developed for inspecting gas mains, and such apparatus is usually referred to as a pipe line "pig". Such pipe line "pigs" typically include a housing with a plurality of sensors mounted on the outer surface thereof in a pre-determined configuration or array to contact the inner surface of the gas main. Since the sensors are in a pre-determined configuration or array, one hundred percent inspection of the wall of the gas main is generally not possible, i.e., the portion of the wall between two adjacent sensors is typically not inspected. Achieving one hundred percent inspection of the wall of the gas main by utilizing such apparatus is generally not possible, and to approach one hundred percent inspection of the wall would require an inordinate number of sensors.
In view of the foregoing, it has become desirable to develop apparatus that provides one hundred percent inspection of the walls of gas mains under operating flow conditions and a method of assessing the serviceability of buried gas mains and recording the data for future reference and analysis. With this capability, natural gas utilities can monitor the rate of degradation of gas mains and plan repairs and/or replacements before failures occur.